International Technical Symposium on Navigation and Timing ENAC, 17 Nov 2015 BeiDou Next Generation Signal Design and Expected Performance Challenges and Proposed Solutions Zheng Yao Tsinghua University 1
The views and opinions expressed in this presentation are those of the author and do not necessarily reflect the official policy or position of any agency of the funding organizations. 2
Outline 1 Background and Motivation 2 Constraints and Challenges 3 New Signal Structures for BeiDou 4 Some Test Results 5 Summary 3
1 Background and Motivation Development of GNSS Worldwide Modernization of existing systems: GPS, GLONASS Construction of emerging systems: Galileo, BeiDou Development of regional systems: QZSS, IRNSS 4
1 Background and Motivation BeiDou Navigation Satellite System Three-step Plan First Step: BeiDou Phase I, 2000~2012 Experimental system SVs: 3 GEO satellites Signals: L+S Band Coverage: regional (China and its surrounding areas) Status: closed 5
1 Background and Motivation BeiDou Navigation Satellite System (cont.) Second Step: BeiDou Phase II, by 2012 Regional System SVs: 14 satellites in orbit (5GEO+5IGSO+4MEO) Signal: B1, B2, B3 (L Band) Coverage: China and its surrounding areas Status: FOC 6
1 Background and Motivation BeiDou Navigation Satellite System (cont.) Third Step: BeiDou Phase III, by 2020 Global System SVs: 35 satellites (5GEO+30nonGEO) Signals: New B1, B2, B3 (L Band) Coverage: global Status: under construction 7
1 Background and Motivation Existing BeiDou Phase II (regional) Signals Two services: authorized service and open service Two open service signals:b1(i), B2(I) BPSK(2), 2.046Mcps OS and AS signals (B1Q & B2Q) are in quadrature The ICD of B1I and B2I released in 2012, 2013 Component Carrier Frequency (MHz) Chip Rate (cps) Bandwidth (MHz) Modulation Type Service Type B1(I) 2.046 OS 1561.098 4.092 QPSK B1(Q) 2.046 AS B2(I) 2.046 OS 1207.14 24 QPSK B2(Q) 10.23 AS B3 1268.52 10.23 24 QPSK AS 8
1 Background and Motivation Signal Design for BeiDou Phase III (global) The design of open service signal is the one of a series of challenges in BeiDou global system construction. In the past years, signal design aroused wide attention from academia and industry in China, and a number of research results has achieved. 9
Outline 1 Background and Motivation 2 Constraints and Challenges 3 New Signal Structures for BeiDou 4 Some Test Results 5 Summary 10
Top-level Design of Signal Architecture New Signals of BeiDou Phase III in the Plan Two different services will provided: authorized service and open service At least two distinct signals for open service will be deployed in L band: new B1 and new B2 New open service signals in the plan B1C: 1575.42MHz (L1/E1) B2: B2a 1176.45 MHz (L5/E5a), B2b 1207.14 MHz (E5b) 11
Where and How GNSS Signals Will be Used? GNSS is a large infrastructure system Long construction cycle Long expected service life Impossible to accurately predict the future The requirement from different applications may be conflicting 12
Typical Receiver Modes High-end Mode (HM) Accuracy is most important while realization complexity is secondary Computationally intensive algorithms are acceptable as long as accuracy benefit can be obtained Low-end Mode (LM) Sensitive to complexity (cost and power consumption) and sensitivity Accuracy is not critical Mid-end Mode (MM) Cost and power consumption are important but restrictions are not as rigorous as in low-end mode Balance between performance and complexity 13
Requirements of Different Modes LM Rx Single frequency processing Higher carrier frequency, lower ionospheric correction error Narrow band receiving with relatively simple processing strategy HM Rx & a part of MM Rx Dual-frequency processing estimates ionospheric group delay Multipath and thermal noise become the major error sources Signals with wider RMS bandwidths are desired For some single frequency receivers which can obtain differential correction information, wider RMS bandwidth is also desired 14
Feature Requirements of B1C B1C Feature Requirements Has multiple possible processing strategies for diversified applications Wide RMS bandwidth to improve inherent accuracy and anti-multipath ability Narrowband low complexity processing strategy for low-end receivers Compatibility and Interoperability with GPS and Galileo Representative Users LM: Consumer electronics, Sensors in IoT, ect. HM/MM: Primary signal in dual-frequency Rx. Geodesy, precise engineering surveying, and precision agriculture etc. 15
Feature Requirements of B2a/B2b Feature Requirements Representative Users Works along with B1C, optimized for high precision ranging measuring B2 Wide RMS bandwidth to improve inherent accuracy and anti-multipath ability Robust to enable tracking in challenging environments HM: Primary signal in dual-frequency Rx. Geodesy, precise engineering surveying, and precision agriculture etc. Compatibility and Interoperability with GPS and Galileo 16
Challenges in GNSS Signal Design Main Challenges in GNSS Signal Design The primary contradiction between performance improvement vs. resource limitation Accuracy Sensitivity Robustness Diversity Performance Improvement VS Resource Limitation Transmitter power Spectral resource Tx&Rx complexity Limitation of spectrum and payload power Unrealistic to broadcast multiple signals each specialized for a specific types of application One challenge is designing flexible signals Allow using different processing strategies to meet the requirements of both LM and HM applications 17
Challenges in GNSS Signal Design Additional Challenges Coexistence of legacy and new signals / Smooth transition from Phase II to Phase III Compatibility and interoperability between multiple GNSSs 18
Outline 1 Background and Motivation 2 Constraints and Challenges 3 New Signal Structures for BeiDou 4 Some Test Results 5 Summary 19
3.1 Quadrature Multiplexed BOC B1C signal should meet varied requirements simultaneously Inherent high ranging accuracy Low-complexity processing strategies Good interoperability with GPS L1C and Galileo E1 OS Quadrature Multiplexed BOC (QMBOC) [1] BOC(1,1) and BOC(6,1) are modulated on two quadrature phases, with same PRN code or different PRN codes I d ( ) = d ( ) ( ) QMBOC ( ) Q = 1 γc ( t) d( t) s ( ) ( t) ± j γc ( t) d( t) s ( ) ( t) s t c t d t s t d BOC nn, d BOC mn, t [1] Zheng Yao, Mingquan Lu, and Zhenming Feng, Quadrature multiplexed BOC modulation for interoperable GNSS signals, Electronics Letters, 2010, 46(17): 1234-1236. 20
Different MBOC Implementations Time-multiplexed BOC (TMBOC) Combining BOC(1,1) and BOC(6,1) in time multiplexing way Composite BOC (CBOC) Weighted summation of BOC(1,1) and BOC(6,1) t Quadrature multiplexed BOC (QMBOC) BOC(1,1) and BOC(6,1) are modulated in phase quadrature with each other I Q t 21
Typical Receiving Strategies Narrowband receiving For low-complexity receivers, QMBOC can be treated as BOC(1,1) Common channel can be used to process B1C, L1C and E1 OS signals Interoperability with GPS and Galileo Wideband Receiving For wideband receivers, QMBOC can be received and processed with full-band. Better performance in anti-multipath Similar baseband process with GPS L1C and Galileo E1 OS 22
Acquisition Performance 0 Correlation Output SNR Loss (db) -0.5-1 -1.5-2 TMBOC & QMBOC TMBOC, BOC 11 -like TMBOC & QMBOC Matched Processing QMBOC, BOC 11 -like -2.5 0 5 10 15 20 25 30 35 40 Bandwidth (MHz) QMBOC vs. TMBOC In matching receiver, QMBOC and TMBOC have the same performance 23
Acquisition Performance 0 Correlation Output SNR Loss (db) -0.5-1 -1.5-2 TMBOC & QMBOC TMBOC, BOC 11 -like QMBOC: 0.56 db TMBOC: 1.12 db QMBOC, BOC 11 -like -2.5 0 5 10 15 20 25 30 35 40 Bandwidth (MHz) QMBOC vs. TMBOC In BOC 11 -like receivers, QMBOC has better performance QMBOC: SNR loss -0.56 db (=29/33) TMBOC: SNR loss -1.12 db (=(29/33)^2) in BOC(6,1) slot, there is no signal, but noise can enter correlator 24
Tracking Performance Equivalent RMS bandwidth Local de-spreading signal waveform dependent Equivalent RMS Bandwidth (MHz) 4 3.5 3 2.5 2 1.5 1 0.5 TMBOC & QMBOC TMBOC, BOC 11 -like QMBOC, BOC 11 -like QMBOC vs. TMBOC 0 0 5 10 15 20 25 30 35 40 Bandwidth (MHz) With MBOC local replica and a wide front-end bandwidth, both QMBOC and TMBOC can have a high tracking accuracy In BOC 11 -like receivers, with wide bandwidth, QMBOC is superior to TMBOC 25
3.2 Asymmetric Constant Envelope BOC Asymmetric Constant Envelope BOC (ACE-BOC) A general deal-frequency multiplexing/ modulation technique Combine 4 or fewer codes with arbitrary power splitting into a spectrum-split constant envelope signal Higher proportion of the transmission power can be allocated to pilot channels or primary service components Q s LQ s UQ Any component can be halted without influencing the constant envelope property I s LI f s UI [1] Zheng Yao, Jiayi Zhang, and Mingquan Lu, ACE-BOC: Dual-frequency constant envelope multiplexing for satellite navigation, IEEE Trans. on Aerospaces & Electronic Systems, Volume 52, Issue 2, 2016 [2] Zheng Yao, and Mingquan Lu, Dual-frequency constant envelope multiplex with non-equal power allocation for GNSS, Electronics Letters, 2012, 48(25): 1624-1625. 26
Typical Embodiments of ACE-BOC Q s LQ s UQ Case 1: Allocating more power to pilot channels to improve the acquisition and tracking performance I s LI s UI f Q Q s LQ s UQ s UQ I s LI Case 2: augment the power of one sideband s UI f I s LI s UI Case 3: Deal with the smooth transition 27 issue in system updating f
3.2 Asymmetric Constant Envelope BOC The scheme for BeiDou B2 is a specific implementation of ACE-BOC. PRN rate 10.23Mcps, subcarrier rate 15.345MHz B2a and B2b have equaled power On each sideband, data : pilot = 1 : 3, modulation phase is orthogonal Q B2a-Q B2b-Q 28 I B2a-I B2b-I f
3.2 Asymmetric Constant Envelope BOC Main Advantages of ACE-BOC More power in pilot, acquisition and tracking performance is improved by 1.8dB Interoperability with GPS L5 and Galileo E5 All of these three signals can be treated as QPSK(10) and share the same channel structure Q Flexible processing strategies s LQ s UQ Fullband matched receiving (FMR) Independent matched receiving (IMR) BPSK-like receiving (BLR) I s LI s UI f 29
Flexible Processing Strategies Fullband matched receiving (FMR) Entire ACEBOC signal can be as the local replica, make the best use of the signal power, but with the highest processing complexity Independent matched receiving (IMR) Receive every signal component separately with local replica of multi-level subcarrier BPSK-like receiving (BLR) Treated as two QPSK signals and received separately Q s LQ s UQ Requires less front-end bandwidth and processing complexity The most commonly accepted receiving mode I s LI s UI f 30
Flexible Processing Strategies 2.5 Multipath Running Average Error (m) 2 1.5 1 0.5 FMR IMR BLR 0 0 0.5 1 1.5 Delay (code) [1] Zheng Yao, Jiayi Zhang, and Mingquan Lu, ACE-BOC: Dual-frequency constant envelope multiplexing for satellite navigation, IEEE Trans. on Aerospaces & Electronic Systems, Volume 52, Issue 2, 2016 31
Outline 1 Background and Motivation 2 Constraints and Challenges 3 New Signal Structures for BeiDou 4 Some Test Results 5 Summary 32
4 Some Test Results Main items in performance analysis and evaluation Acquisition sensitivity Tracking sensitivity Ranging accuracy Demodulation performance Interference resistance Compatibility. A series of performance evaluation has been carried out for each signal structure option of BeiDou global system Theoretical analysis Computer simulations in signal-level Ground test with signal simulator and receivers prototype Satellite-to-receiver validation 33
4 Some Test Results GNSS signal simulation and evaluation system Signal Generator (Baseband Unit +VSG) GNSS Receiver
4 Some Test Results A Case Study Tracking Performance:ACE-BOC vs AltBOC ACE-BOC and AltBOC with same total transmit power BPSK-like receiving strategy, sharing channel Acquisition and tracking channels for them are totally the same Pilot component tracking, aiding data demodulation 35
Power Ratio AltBOC:Pilot/Data = 1:1 ACE-BOC:Pilot/Data = 3:1 Under the same total transmit power, ACE-BOC has a higher pilot C/N0, but relative lower data C/N0 Signal PRN Status Lock Value Pilot C/N0 Data C/N0 Doppler ACE-BOC has a higher loop locked Value of pilot, more robust tracking
In the same transmit power conditions, AltBOC loop has lost lock, while ACE- BOC remained stable tracking Signal PRN Status Lock Value Pilot C/N0 Data C/N0 Doppler
Code Tracking Error Standard Deviation (m) 1 0.8 0.6 0.4 0.2 0 ACE-BOC B2a AltBOC B2a -145-142 -131-128 -126 Signal Power (dbm) 38
Outline 1 Background and Motivation 2 Constraints and Challenges 3 New Signal Structures for BeiDou 4 Some Test Results 5 Summary 39
5 Summary As new generation GNSS evolves, a number of new signal structures are emerging Although GNSS future cannot be predicted accurately, the inherent various receiving strategies can make signals construct multiple balance points between complexity and performance for both transmitters and receivers, thus provide more diversified choices for system designers as well as users According to the system construction requirements, two novel GNSS signal structures, QMBOC and ACE-BOC, are proposed for China s BeiDou global navigation system. 40
5 Summary QMBOC is new signal structure with better performance and flexible receiving techniques. ACE-BOC is a general modulation/ multiplexing technique, which allows the combination of 4 or fewer signal components with any power allocation. The scheme for BeiDou B2 is a specific implementation of ACE-BOC, with significant performance improvement, and diversified receiving strategies. Both of these two signal structures have good compatibility and interoperability with GPS and Galileo. 41
Thank you for your attention! 43